A convenient method for delivering electrical power to electrically powered trains is by means of an overhead catenary system suspended above the track between support columns or other supporting structures spaced along the track. A typical catenary system, referred to hereinafter simply as a catenary, comprises a contact wire suspended on hanger elements (usually bronze rods) from a messenger wire which is attached to the support columns. The contact wire is maintained at a high electrical potential and supplies electrical current to power the train. The train has an electrical pick-up structure or pantograph mounted on its roof. The pantograph typically comprises an elongated beam mounted transversely to the direction of travel and has a contact surface for engaging the contact wire of the catenary. The pantograph is in substantially continuous contact with the contact wire as the train travels along the track. The contact wire is suspended substantially parallel to the track while the messenger wire traces a curve, known as a catenary, between the support columns. A catenary curve is defined as the curve assumed by a flexible cord or chain of uniform density which hangs freely from two fixed points and approximates a parabola. Although the messenger wire curves under the pull of gravity the contact wire is maintained parallel to the track by varying the length of the hangers which suspend the contact wire from the messenger wire. An analogous catenary system can be observed in the structure of a suspension bridge, the main support cables assuming a catenary shape between the support towers while the bridge deck is suspended substantially level between the towers.
The position of the catenary above the track must be carefully controlled and maintained to ensure continuous contact between the pantograph and the contact wire for the continuous delivery of electrical power to the train. Not only must there be substantially continuous contact, the point of contact on the pantograph must be continuously varied along the length of the pantograph as the train moves along the track to prevent excessive frictional wear at any one point on the pantograph. Thus both the vertical and horizontal positions of the contact wire relative to the pantograph are important to the efficient functioning of the train.
Control of the vertical position ensures that the proper contact pressure is maintained. If the wire is too low contact pressures may be excessive causing excessive frictional wear of the wire and pantograph. The contact wire could also become snared on a part of the train beneath the pantograph, bringing down the catenary and electrocuting passengers or bystanders. If the wire is too high, the pantograph may tend to lose contact with the wire depriving the train of power and stranding the train. If the contact is intermittent the train will be subject to power surges yielding an uncomfortable ride and potentially damaging the equipment.
Controlling the horizontal position of the catenary relative to the pantograph keeps the contact wire from laterally disengaging from the pantograph as the train moves down the track. The horizontal position of the catenary must be maintained so as not to extend laterally left or right of the train beyond the physical extent of the pantograph. However, the catenary must not contact the pantograph continuously in one spot. If the system were designed to maintain contact at one spot on the pantograph, then the pantograph would rapidly wear out due to the friction between the contact wire and the pantograph, the wire would literally saw through the pantograph. To avoid this problem it has been found advantageous to arrange the catenary above the track in a zig-zag manner about the track centerline. The contact point between the wire and the pantograph is thus varied along the length of the pantograph as the train moves down the track, and the contact surface of the pantograph is subjected to even wear. The zig-zag pattern is carefully controlled however to ensure that the contact wire remains laterally within the bounds of the pantograph.
Maintaining the vertical and horizontal position of the catenary relative to the pantograph requires the capability to measure two parameters of the catenary relative to a plurality of predetermined reference points fixed at a plurality of predetermined locations along the track. For convenience the centerline of the track at the top of the rails is often used as the reference point at each predetermined location along the track where measurements are taken. The two parameters measured are termed the height and stagger of the catenary.
Height, when used in this context, refers to the perpendicular distance between a plane parallel to the track through the reference point and the contact wire of the catenary at the predetermined location along the track. Thus, for level track, the height will simply be the vertical distance between the reference point and the contact wire. If the track is banked, as would be done for curved track, the height will not be the vertical distance but will be measured at an angle to the vertical corresponding to the bank angle. Height is thus always measured perpendicularly to the plane of the track regardless of the angle the track makes with the horizontal.
Stagger, when used in the context of this invention, refers to the lateral offset of the catenary from the track centerline as measured perpendicularly to the height. For example, on level track, the stagger is the horizontal distance of the contact wire from the track centerline. Height and stagger always form a right angle, and a right triangle is formed by the height, stagger and the line of sight (LOS) distance from the reference point to the contact wire. Height and stagger are related to the line of sight distance analogously to the legs of a right triangle to the hypotenuse by the trigonometric functions of cosine and sine respectively.